The present invention relates to a semiconductor device comprising thin semiconductor films, and to a method for producing it. In particular, it relates to technology concerning the constitution of bottom-gate-type TFT.
The terminology xe2x80x9csemiconductor devicexe2x80x9d referred to herein is directed to any and every device that functions on the basis of semiconductor characteristics. Accordingly, TFT, semiconductor circuits, electro-optical devices and electronic instruments referred to herein shall be all within the category of that terminology, semiconductor device.
With great popularization of notebook-type personal computers, portable information terminals, etc. in recent years, there is increasing a great demand for active matrix-type liquid crystal devices (AMLCD). For those, techniques of producing thin film transistors (TFT) from thin semiconductor films formed on insulating substrate are being much promoted.
Thin film transistors are grouped into top-gate-structured ones (typically, planar-structured ones) and bottom-gate-structured ones (typically, inverse stagger-structured ones). Of those, widely noticed are AMLCD with circuits of bottom-gate-type TFT of which the structures are simple and the productivity is high.
At present, techniques for producing inverse stagger-type TFT comprising forming polysilicon films on a glass substrate are being developed, and some of them are being industrialized to produce commercial products.
For forming a polysilicon film on a glass substrate, generally employed is a means of crystallizing an amorphous silicon film through laser exposure. However, one problem with the laser crystallization is that polysilicon films with uniform crystallinity are difficult to obtain.
The crystallinity as referred to herein will be typically indicated by the grain size of crystals. The crystals of polysilicon films as formed through laser crystallization generally have relatively uniform grain sizes, but, as the case may be, their grain sizes will often vary depending on delicate changes in the starting amorphous silicon films (temperature, shape, etc.).
For example, in producing inverse stagger-type TFT, a gate-insulating film and an amorphous silicon film are laminated on a gate electrode in that order, and the resulting laminate is subjected to laser crystallization. Through our experiments, we, the present inventors have verified that, in the step of laser crystallization, the crystals of the polysilicon film formed just above the tapered area of the gate electrode shall have extremely small grain sizes and are therefore in a semi-crystalline condition.
The area just above the tapered area of the gate electrode is the most important region for the junction of a channel-forming region and a drain region, the junction of a channel-forming region and an LDD region or the like that governs the characteristics of TFT, and the polysilicon film as formed through the conventional laser crystallization shall have non-uniform grain sizes in that most important region, by which the characteristics of TFT are worsened.
The present invention has been made in consideration of the problems noted above, and its object is to provide a technique for forming a polysilicon film having a highly uniform grain size at least in the channel-forming region and the impurity-poor.region in TFT
Gate electrodes made from a material consisting essentially of aluminium (Al) or copper (Cu) are favorable for a matrix circuit to be formed on a large-sized glass substrate, as their wiring resistance is be extremely small. Therefore, such a low-resistance material will be much more used in future for TFT wiring (including gate electrodes).
However, for example, when gate electrodes of aluminium are formed in inverse stagger-type TFT, the angle at their taper could be at most 60 xc2x120xc2x0. Therefore, as compared with those of any other materials, the aluminium gate electrodes are disadvantageous in that the active layers to be formed thereover shall have a great difference in level.
Experimental data have been disclosed that indicate that, in the step of laser crystallization of an amorphous silicon film, the film just above a gate electrode consisting essentially of aluminium or copper differs from that in the other area in the rate of heat absorption. This is because the gate material consisting essentially of aluminium or copper has high heat conductivity. Accordingly, the temperature of the film just above the gate electrode is locally lowered due to the heat-absorbing effect of the gate electrode.
In that situation, we, the present inventors have considered that the gate electrode, if having a large taper angle on a substrate, will produce a definite boundary of two regions of the overlying amorphous silicon film each having a different rate of heat absorption, thereby causing the great temperature change profile around the boundary during the crystallization of the film, and, as a result, the crystals of the polysilicon film as formed just above the tapered area of the electrode will have small grain sizes and will therefore be in a semi-crystalline condition.
The tapered area (edge) of the gate electrode is an extremely important region, as corresponding to the edge of a channel-forming region (that is, the source/drain junction). Therefore, it is desired that at least the polysilicon film to constitute a channel-forming region and an impurity-poor region does not have a crystallinity-disordered region such as that noted above.
As having been so mentioned hereinabove, the amorphous silicon film to be crystallized must have a uniform temperature distribution during the step of laser exposure in order that the crystals of the resulting polysilicon film to be a channel-forming region and a low concentration impurity region may have uniform grain sizes (or that is, the film to be such regions may have uniform crystallinity).
In order to attain the object noted above, one aspect of the present invention is to provide a semiconductor device comprising at least;
a gate electrode formed on a substrate having an insulating surface,
a heat relaxation layer formed to cover said gate electrode,
a gate-insulating layer formed on said heat relaxation layer, and
a polysilicon film formed on said gate-insulating layer;
wherein the heat conductivity of said heat relaxation layer is lower than that of said gate electrode.
Another aspect of the invention is to provide a semiconductor device comprising at least;
a gate electrode formed on a substrate having an insulating surface,
a heat relaxation layer formed to cover said gate electrode,
a gate-insulating layer formed on said heat relaxation layer, and
a polysilicon film formed on said gate-insulating layer;
wherein the heat conductivity and the electric conductivity of said heat relaxation layer are lower than those of said gate electrode.
In the constitution of the invention, the heat relaxation layer plays an extremely important part. Concretely, the heat propagation is relaxed by the heat relaxation layer, whereby the heat-absorbing effect of the gate electrode is attenuated to thereby make the energy of laser beams having been applied to an amorphous silicon film absorbed more efficiently by the amorphous component of the film.
Still another aspect of the invention is a method for producing a semiconductor device, which comprises;
a step of forming a gate electrode on a substrate having an insulating surface,
a step of forming a heat relaxation layer to cover said gate electrode,
a step of forming a gate-insulating layer on said heat relaxation layer,
a step of forming an amorphous silicon film on said gate-insulating layer, and
a step of exposing the amorphous silicon film to laser beams or to intense light of which the intensity is equivalent to that of laser beams, to thereby crystallize the film into a polysilicon film;
the method being characterized in that the heat conductivity of said heat relaxation layer is lower than that of said gate electrode.
Still another aspect of the invention is a method for producing a semiconductor device, which comprises;
a step of forming a gate electrode on a substrate having an insulating surface,
a step of forming a heat relaxation layer to cover said gate electrode,
a step of forming a gate-insulating layer on said heat relaxation layer,
a step of forming an amorphous silicon film on said gate-insulating layer,
a step of keeping or adding a crystallization-promoting catalyst element on said amorphous silicon film,
a step of heating said amorphous silicon film to thereby crystallize it into a polysilicon film, and
a step of exposing the polysilicon film to laser beams or to intense light of which the intensity is equivalent to that of laser beams, to thereby improve the crystallinity of said polysilicon film,
the method being characterized in that the heat conductivity and the electric conductivity of said heat relaxation layer are lower than those of said gate electrode.